1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In order to reliably obtain a desired image density in image forming apparatuses using an electrophotographic method, such as a copier and a laser beam printer, toner patterns including toner patches which are test images for different toner densities are formed on the image bearing member. The density of the toner patch is detected by an optical detector. Based on the result detected by the optical detector, imaging conditions, for example, a toner density, an LD power, a charging bias, and a developing bias, are adjusted so that the target toner density is obtained.
Generally, such adjustment of the toner density is performed after the power is turned on or at certain intervals after completion of a print job. In order to stabilize the image density during continuous imaging operation, the toner pattern is formed at specific times, such as between successive images to be printed, and is detected by the optical detector to adjust the toner density.
In the image forming apparatus using an intermediate transfer method, the toner patch formed on the image bearing member such as a photoconductor is transferred onto an intermediate transfer member, and then the toner patch is detected by the optical detector.
In such an image forming apparatus, a recording medium is transported to a secondary transfer area between the intermediate transfer member and a secondary transfer member where the toner image on the intermediate transfer member is transferred secondarily onto the recording medium. The toner patch is formed between successive images so that the toner patch does not contact the recording medium in the secondary transfer area to prevent toner adhered to the toner pattern from sticking to the recording medium.
Although contamination of the recording medium can be prevented, as the toner patch passes through the secondary transfer area, in the absence of the recording medium the toner patch instead contacts undesirably the secondary transfer member disposed opposite the intermediate transfer member in the secondary transfer area. As a result, the secondary transfer member gets contaminated by the toner of the toner patch, and then the toner adhered to the secondary transfer member sticks to a rear surface of a subsequent recording medium.
To address this problem, sheet detector can be disposed upstream from the secondary transfer area in the direction of conveyance of the recording medium to detect the recording medium. When the sheet detector detects the recording medium, the secondary transfer member is separated from the intermediate transfer member. In this configuration, the secondary transfer member contacts the intermediate transfer member only when the recording medium enters the secondary transfer area, thereby preventing the toner of the toner pattern adhered to the intermediate transfer member between the successive images on the recording medium from sticking to the secondary transfer member.
Although advantageous, during continuous printing, the secondary transfer member needs to contact and separate from the intermediate transfer member repeatedly between successive recording media sheets. Generally, in order to increase productivity during continuous printing, the imaging speed is increased and/or a distance between successive recording media sheets is reduced.
More specifically, the secondary transfer member needs to be separated from the intermediate transfer member after the trailing edge of the preceding recording medium passes through the secondary transfer area, but needs to contact again the intermediate transfer member before the front edge of the subsequent recording medium arrives at the secondary transfer area, thereby complicating efforts to increase imaging speed and hence productivity for continuous printing operation.
To counteract such difficulty, the moving speed of the secondary transfer member separating from and contacting the intermediate transfer member may be increased. However, increasing the speed of movement of the secondary transfer member intensifies vibration, hence adversely affecting imaging operation.
Although the problems described above relate to the movement of the secondary transfer member relative to the intermediate transfer member, similar problems may occur in an image forming apparatus using a direct transfer method in which the toner image is transferred directly from an image bearing member such as the photoconductive member onto the recording medium in a transfer area between and the image bearing member and the transfer member.
Furthermore, the recording medium may be contaminated when degraded toner in the developing device is discharged forcibly onto the image bearing member between successive images (toner images) when degraded toner is replaced with fresh toner.
There is thus demand for an image forming apparatus capable of preventing contamination of the recording medium without degrading productivity during continuous printing.